[jmstar09]

I have had to fix at least ten of those over the years and what I do is to pick out all the old plastic "Glass" and then clean the entire window with brake cleaner to remove any trace of brake fluid in the window then I fill the entire window with RTV silicone. Black, Red, Clear, whatever. Then I use a matchbook cover or a putty knife to smooth off the surface in all four axis. N,S, E, W so the window gets sealed well all the way around. A little will ooze through the holes in the metal plate toward the inside of the reservoir. You can smooth it out with a pinky finger or just leave it. I leave it. Then allow the silicone to dry for 24 hours, refill and bleed and you are done. If you want to check the fluid you have to open the reservoir cap.

I suppose you could use RTV to glue a new sight glass in place. The RTV will stand up to brake fluid but it's not easy to keep the job neat.

I've also had a bunch off scooters in the shop that had sight glass that was getting ready to leak/fail due to sun exposure. I smeared a nice coat of RTV over the top of the existing glass.